1. Field of the Invention
This invention pertains to modified zeolite catalysts useful for selective alkylation of aromatic molecules and selective disproportionation of mono- and di-alkylaromatics. It is particularly directed to maintenance of the high para-selectivity of such modified catalysts.
2. Description of the Prior Art
Modification of certain types of crystalline zeolite catalysts by deposition of phosphorus and magnesium thereon is known to cause a desirable increase in the selectivity to the para isomer when the modified catalyst is utilized to promote alkylation of aromatic molecules and/or disproportionation reactions of mono- and di-alkylaromatic molecules. Such catalysts are disclosed in U.S. Pat. Nos. 4,011,276 to C. Chu and 4,128,592 to W. W. Kaeding, for example.
It has been found, however, that the desirable para selectivity characteristics of these catalysts rapidly deteriorate on scale-up of the reaction from laboratory-scale reactors to commercially viable large-scale reactors. While the overall activity of the modified catalysts remains high (i.e. the overall alkylation or disproportionation activity), the selectivity to the expected para isomers drops off at an alarming rate. In some cases this "aging" of the selective characteristics of these catalyst has been reported to be as high as 20% or more per day in the reactor.